Heat exchangers such as condensers, radiators, evaporators, heater cores, and coolers made of aluminum or aluminum alloy or other metals are widely used today. These heat exchangers generally include perforated fins brazed to the external surface of tubes and plates that form the structure of the heat exchanger. The tubes are usually extruded, welded, or folded and the fins are usually made from coils of sheet stock.
Brazing is a process that involves the joining of components into an apparatus using a brazing filler material whose melting point is lower than that of the components. The brazing process is typically used to join components that are either metals or alloys. Typically the brazing filler material is placed adjacent to or in between the components to be joined or it is clad to one of the components, the components are assembled into the apparatus, and the assembled apparatus is then heated to a temperature where the brazing filler material melts but not the components. Upon cooling, the brazing filler material forms a metallurgical bond between the surfaces of the components.
Prior to assembly into heat exchangers the tubes and plates are typically coated or plated with a corrosion protector using known techniques such as twin-wire arc thermal spraying. Zinc or zinc-aluminum alloys are generally used as a corrosion protector, but any known corrosion inhibitor may be used. The tubes are typically prepared prior to assembly to carry the brazing filler or cladding material that fills the joints between the tube and fins during brazing. The brazing filler is typically applied to thin sheet stock as a cladding layer in the form of an overlayed sheet that is rolled and bonded to the aluminum tube stock. The cladding comprises a material or materials known in the art to be capable of melting at a temperature lower than the heat exchanger components such as an aluminum-silicon alloy so that, during brazing, the cladding will form brazed joints. The use of such clad brazing sheets is well known. Other methods of applying the cladding material include spraying it on or dipping one of the components into a cladding solution.
Prior to brazing of aluminum heat exchangers, tube cladding and plate surfaces must be cleaned and de-oxidized. Removal of the oxidation layer is necessary to form strong joints. This is generally accomplished using a material commonly known as flux that chemically cleans and de-oxidizes the surfaces and protects the aluminum from further oxidation. Typical brazing fluxes contain chlorides and/or fluorides and the flux material typically melts at a lower temperature than that of the brazing filler material. Once molten, the brazing flux material works to dissolve the hard shell of metal oxides on the two surfaces, which enhances the wetting and flow of the molten brazing filler material, thereby allowing it to be drawn freely by capillary force between the joints of the components to be brazed. The composition of the brazing filler material is determined by the composition of the surfaces to be joined as is known by ordinary skill in the art. Likewise, there are numerous brazing flux materials that are available and the particular one utilized depends on the identity of the components to be brazed.
In current manufacturing processes, typically the brazing filler material is applied to one of the surfaces to be joined and the components are pre-assembled into the apparatus. A braze bar carrier system is typically used to hold the components together and in alignment during application of the flux material and during passage through the brazing furnace. After pre-assembly the entire apparatus and carrier system is often dipped in a water-flux slurry or such a water-flux slurry is sprayed onto the entire assembly. Alternatively, the flux material is applied to the entire apparatus via a static dry powder process. As discussed above, the flux is actually only required at the localized areas where the two surfaces are being joined. The entire fluxed apparatus and carrier system are then passed into a brazing furnace where the brazing flux material and filler become liquid and flow between the parts. Once cooled the cladding fuses the parts of the apparatus together.
Prior art carrier systems often comprise brazing trays, frames or bars. Typically these components are formed from stainless steel tubing, turned on edge. The brazing carrier systems are often coated with various paints or other coatings like boron-nitride or green stop off. These coatings are designed to serve as a barrier between the stainless steel surface and the brazing flux material. As these coating materials wear off flux that lands on the stainless steel surface starts to wet the surface thereby allowing the brazing cladding material to braze the trays, frames, and bars to the components being joined. When this occurs this can cause holes and damage to the apparatus when the carrier system parts are removed from the apparatus. This results in quality and scrap problems. In addition, with frequent use these coating materials must typically be reapplied to the carrier system parts on a monthly basis. Thus, it would be advantageous to develop a carrier system that avoided or eliminated the problem of brazing of the carrier system onto the apparatus being carried.
One prior art solution is disclosed in U.S. Pat. No. 6,199,742. In this patent a special tooling base is made of graphite to provide for a controlled co-efficient of thermal expansion between the tooling base and the part being brazed. The disclosed tooling utilizes a series of slip sheets having lubricants such as stop-off compounds and others to prevent sticking of the component being brazed to the tooling. The part to be assembled is received on the slip sheet which is placed on the graphite tooling base. Additional slip sheets are placed on top of the apparatus to be brazed each of the slip sheets including lubricants such as stop-off. Finally pressure is applied to the parts to be brazed in the form of a pan filled with heavy metal pellets. While this disclosure provides some protection of the tooling by utilization of the slip sheets it still requires additional components that need to be replaced and can be subject to wear leading to scrap parts prior to their replacement. In addition, the complicated arrangements of slip sheets can make assembly of the unit difficult. An earlier solution to the problem of adherence of the fixtures to the components being brazed is disclosed in U.S. Pat. No. 5,549,238. In this patent the brazing tray surface in contact with the parts to be brazed comprises a layer of ceramic material. The ceramic material provides a non-stick surface for preventing flux from adhering to the brazing tray. The difficulty with this solution is it can be hard to coat various materials uniformly with a ceramic coating and such coating is fragile and subject to wear. In addition, the non-stick coating cannot be easily replaced and in fact the entire tray must be scraped once the coating has become worn.
Thus there exists a need to provide a brazing fixture having a non-stick surface that is easily replaced and subject to less wear than those found in the prior art.